Resources are based on resource types and are available from resource providers. There are numerous providers available in Azure, which provide the resource types needed by users to create their instances. For example, the Microsoft.Compute resource provider provides virtual machine resource types. Using virtual machine resource types, instances of virtual machines can be created.
Resource providers are required to be registered with Azure subscriptions. Resource types will not be available in a subscription if resource providers are not registered. To get a list of the providers that are available, or to see the ones that are registered and the ones that aren't, or even to register non-registered providers, this dashboard can be used: